00b4952cea51940af2000000

Chinese Science Bulletin

2007 SCIENCE IN CHINA PRESS

Springer

www.scichina.com www.springerlink.com Chinese Science Bulletin | January 2007 | vol. 52 | no. 1 | 1-?

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Advances in Unmanned Aerial Vehicles Technologies Agus Budiyono1 1 Smart Robot Center, Department of Aerospace Information Engineering, Konkuk University, Seoul, Korea. Previously with Center for Unmanned System Studies, Institut Teknologi Bandung, Indonesia

The utilization of unmanned vehicles has become increasingly more popular today and been successfully demonstrated for various civil and military applications. The unmanned aerial ve-hicles (UAVs) have shown applications in different areas including crop yield prediction, land use surveys in rural and urban regions, traffic surveillance and weather research. The unmanned small scale helicopters are particularly suitable for demanding problems which requires accurate low-speed maneuver and hovering capabilities such as detailed area mapping. Generally a certain level of autonomous flight capability is required for the vehicle to achieve its mission. The basic autonomy level is to maintain its stability following a desired path under embedded guidance, na-vigation and control algorithm. The UAV technology trends indicate that to cope with the more stringent operation requirements, the UAVs should rely less and less on the skill of the ground pilot and progressively more on the autonomous capabilities dictated by a reliable onboard computer system. To systematically develop and enhance flight autonomy, a rotary wing UAV (RUAV) or model helicopter has been proposed and used as a flying test-bed at various major research centers. The ability of the helicopter to operate in the hovering mode makes it an ideal platform for a step-by-step autonomous capability development. On the other hand, a small heli-copter exhibits not only increased sensitivity to control inputs and disturbances, but also a much richer dynamics compared to conventional unmanned aerial vehicles (UAVs). The paper surveys recent advances in modeling, control and navigation of autonomous unmanned aerial vehicles. Without loss of generality, an autonomous small scale helicopter research program is taken as a case study. Approaches to modeling and control for such a vehicle are presented and discussed. Future directions in the advancement of UAV technologies are identified and key barriers hig-hlighted.

Unmanned aerial vehicle, model identification, control, navigation, trajectory generation

I. Introduction

A widely used definition of UAV is an aerial vehicle (in-cluding fixed-wing, rotary-wing or airship platform) which can sustain its flight along a prescribed path without an on-board pilot. The UAV technology has proven applica-tions in many areas such as environmental monitoring and protection, meteorological surveillance and weather re-search, agriculture, mineral exploration and exploitation, aerial target system, airborne surveillance for military land operations, and reconnaissance missions. The unmanned small scale helicopters enjoy no requirement for runway and are particularly suitable for demanding problems such

as traffic or volcanic areas surveillance, detailed area map-ping, video footage recordings and crop dusting or spraying. Table 1 lists applications of contemporary UAVs in differ-ent areas.

A recent progress in the supporting technologies has enabled the development of semi to fully autonomous UAV. This includes the availability of compact, lightweight, af-fordable motion detecting sensors essential to the flight control system and compact lightweight low-cost compu-ting power for autonomous flight control. A wide varieties of autonomous UAV platforms have been developed and flown ranging from fixed-wing to rotary wing platforms,

2 CHEN LiQun et al. Chinese Science Bulletin | Ja??? 2007 | vol. 52 | no. ? | ?-?

several minutes to hours/day in endurance and 100 grams to 800 kg in weight. From all types of UAVs, the small scale rotorcraft-based vehicle has been considered one that exhibits the most complex dynamic properties. From the perspective of control area, the RUAV demonstrates liter-ally all challenges that have attracted enormous interests from industry and academia alike. The challenging prob-lems include higher bandwidth, hybrid modes, non-holonomic, under-actuation, multi input multi output (MIMO), and non-minimum phase. The paper discusses the advances in UAV technologies from the perspective of modeling and control of rotorcraft-based aerial vehicles.

II. Background: Science and Technology

A. Survey of UAVs

The viability of UAV as a multipurpose research vehicle has driven great interest since recent decades. The basic technology building blocks responsible for the current ad-vances include airframes, propulsion systems, payloads, safety or protection systems, launch and recovery, data processor, ground control station, navigation and guidance, and autonomous flight controllers. The following brief survey is focused on the area of navigation, guidance and control of UAVs. Various control design for UAVs has been proposed ranging from linear to nonlinear synthesis, time invariant to parameter varying, and conventional PID to intelligent control approaches. The developed controllers have been implemented for different aerial platforms: air-ship (blimp), fixed-wing UAV, small scale helicopter, quad-rotors, and MAV.

The research on autonomous airship is reported in (Azin-heira, 2008) where the authors proposed a nonlinear control approach for the path-tracking of an autonomous underac-tuated airship. A backstepping controller is designed from the airship nonlinear dynamic model including wind dis-turbances, and further enhanced to consider actuators satu-ration. The hover control using the same approach for such a vehicle is presented in (Azinheira and Moutinho, 2008). A number of investigations have been conducted for con-trol and stabilization of quadrotor UAV. In (Raffo, 2008), a robust control strategy to solve the path tracking problem for such a vehicle was designed in consideration of external disturbances like aerodynamic moments. A state parameter control based on Euler angles and open loop positions state observer was proposed by Mokhtari and Benallegue (2004). The work was continued in (Mokhtari, 2005) in which a mixed robust feedback linearization with linear GH con-troller was applied. An actuator saturation and constrain on

state space output are introduced to analyze the worst case of control law design. A different approach was proposed in (Madani, 2007) where a backstepping control running pa-rallel with a sliding mode observer for a quadrotor vehicle. The sliding mode observer works as an observer of the qu-adrotor velocities and estimator of the external disturbances such as wind and parameter uncertainties. In (Escareno et.al., 2008), the authors proposed a three-rotor configura-tion which incorporates certain structural advantages in order to improve the attitude stabilization. The control strategy is robust with respect to dynamic couplings and to the adverse torques produced by the gyroscopic-effect and propellers drag.

The research on autonomous flight using model helicopters as a test-bed has been performed by a large number of teams all over the world. The MIT UAV team successfully developed an autonomous aerobatic helicopter in (Gravilets, 2003). The development relied on the modeling framework of the miniature helicopter dynamics. A methodology for designing model-based control strategies for autonomous aerobatic maneuver was proposed and validated experi-mentally. Referring to previous work by Mettler (Mettler et.al., 2002) at Carnegie Mellon Robotics Institute, the ba-sis for a simplified modeling framework was considered to stem from the fact that the dynamics of small-scale heli-copters is dominated by the rotor response. The real-time control system was developed using a Hard-ware-In-the-Loop (HIL) simulation system which allows high fidelity representation of the signals time-dependence in real time navigation scheme

At Georgia Tech, the Open Control Platform (OCP)a new object-oriented real time operating software architec-ture has been used onboard the GTMAX UAV helicopter to compensate for the simulated in-flight failure of a low level flight control system. The viability of designing in-expensive architecture, along with a relatively simple pro-cessor, will pave the way for the extremely low-cost flight control and guidance systems. Another novel contribution was the use of Pseudo Control Hedging (PCH) in the adap-tive flight control scheme for improving tracking perfor-mance of a small helicopter. Using this architecture, a con-solidated reference command that includes position, veloc-ity, attitude and angular rate may be provided to the control system. At UC Berkeley, the research on an autonomous helicopter has been conducted as reported in Koo and Sastry(1998), Koo et.al.(2001) and Kim et.al.(2003). A helicopter ma-thematical model is first established with the lump-parameter approach. The control models of the

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RUAVs are then derived by the application of a time-domain parametric identification method to the flight data of target RUAVs. The classical control theory and modern linear robust control theory are applied to the iden-tified model. The proposed controller are validated in a nonlinear simulation environment and tested in a series of test flights (Shim, 2000).

The discussion on this paper is centered on model-based control design and navigation system technology in the framework of recent advances in UAVs elaborated in the following order. In the section below (II), the system and technology background of UAVs are presented including a brief survey of contemporary UAVs, summary of lessons from the research on RUAV modeling and controls, and identification of trends in UAV technology. Section III presents the review of modeling of RUAV using combined first principle and time-domain identification. Nonlinear dynamic modeling is presented based on first principle ap-proach using X-cell 60 small scale helicopter as a test bed. A method for linearization procedure is elaborated to pro-vide an analytical model for the implementation of linear control. Section IV is focused on discussion on simulation, control and guidance for UAV. Some approaches for con-trol synthesis are demonstrated for illustration. The last section (V) identifies emerging technologies in the area of aerial robotic. Concluding remarks on the challenges and future directions are made in final section (VI).

B. Lessons learned from CentrUMS-ITB UAV Program The research on RUAV at the Center for Unmanned Sys-tems Studies (CentrUMS)-ITB was carried out by using a fully instrumented X-cell 60 SE model helicopter similar to one used by MIT team as shown in Fig. 1. The mini heli-copter is characterized by a hinge-less rotor with a diameter of 0.775 m and mass of 8 kg. The X-Cell blades both for main and tail rotors use symmetric airfoils. The vehicle has been used by a number of research centers as published in a number of literatures (Gravilets,2003; Bogdanov,2003; Bogdanov,2004). Therefore comparison and validation can be achieved from the available published results. Using the test bed, studies on modeling and control of RUAV were conducted. A great deal of effort was focused on developing nonlinear model based on first principle ap-proach. The nonlinear model was implemented in Simu-link/Matlab with parameters are measured independently or obtained from literatures. Flight tests were conducted to validate the model. Various control synthesis were studied for performance comparison. The important lesson learnt from the experience is that a small scale helicopter is a in-tricate and unstable platform; to utilize it for a useful re-

search test-bed there is a compelling need for development of mathematical model that capture the key dynamics of the vehicle with reasonable level of complexity for the purpose of control design. A number of key results are pre-sented in Section IV.

Figure 1: Instrumented X-Cell 60 SE- CentrUMS-ITB

C. Trends in UAV Research

More stringent mission requirements have driven the UAVs to have a higher level of autonomy dictated by a reliable onboard computer system. The metric for UAV level of autonomy is given in Table 2 (Sholes, 2006). Some key areas in current state-of-the-art aerial robotic technologies are responsible for enabling AUVs to achieve its required level of autonomy. Current status of UAV research activi-ties in these areas can be summarized as the following:

1. State estimation algorithm. To achieve better perfor-mance, multiple sensors are typically fused together using EKF in a sensor fusion algorithm. Propagated IMU-data can be fused with discrete updates from GPS and altimeter. Several design examples are pro-vided in (Johnson and Kannan, 2002). Recent study include the use of nonlinear adaptive observers for es-timating speed of UAV from IMU measurements only without the aid of GPS (Khadidja, 2007).

2. Simultaneous Localization and Mapping. An un-manned aerial vehicle (UAV) is tasked to explore an unknown environment and to map the features it finds, but must do so without the use of infrastructure-based localization systems such as GPS, or any a priori ter-


rain data. A statistical estimation technique allows for the simultaneous estimation of the location of the UAV as well as the location of the features it sees.

3. Vision for guidance. Computer vision is used as a feedback sensor in a control loop for an autonomous flight system. (Amidi et.al, 1998). More recent exam-ple is precision targeting without using secondary actuation or add-on gimbal system.

4. The use of GPS as attitude sensor. The need for re-duced complexity avionics system has driven the re-search on the use of single GPS for obtaining attitude estimate (Kornfeld, 1998).

5. Integrated modeling. Linear model is obtained by us-ing combination of first principle results and time or frequency domain identification scheme.

6. Trajectory generation using maneuver automaton. Ve-hicle motion is described by library of motion primi-tives (Frazzoly et.al, 2005). The trajectory between two positions and vehicle states is found by searching the sequence of motion primitives which will best sa-tisfy an objective function. One important application of guidance system is collision avoidance between vehicle at its tight and structured environment or be-tween vehicles operating in formation or multi agent system.

7. Safety verification. Safety verification or reachability analysis aims to show that starting at some initial conditions, a systems cannot evolve to some unsafe regions in the state space. Unsafe region for UAV ap-plication can be defined in the context of proximity to obstacles, fuel availability (endurance), un-flyable zone and/or communication range. A new concept called barrier certificate is being used for safety veri-fication of hybrid systems.

III. Modeling of RUAV

The requirement for successful navigation and guidance task is stabilization of vehicle platform. Viewed as a mul-ti-loop system, guidance and navigation is represented by the outer-loop and control and stabilization the inner loop. The design starts from the most inner loop outward. In this context, to control small scale helicopter as unstable plat-form with complex dynamics require sufficiently accurate model. This section elaborates the modeling technique and the corresponding model-based control synthesis.

TABLE I

UAV LEVEL OF AUTONOMY

Level Level D escriptor Perception/Situational A w areness10 Fully A utonom ous Cognizant of all w ithin battlespace

9 Battleship sw arm cognizanceKnow s intent of self and others (friendlyand threat) in a com plex/intenseenvironm ent; on board tracking

8 Battleship single cognizanceProxim ity Inference - intent of self andothers (friendly and threat);Reduced dependence on off-board data

7 Battleship know ledge

Short track aw areness - H istory andpredictive battlespace data in lim ited range,tim efram e, and num bers; Lim ited inferencesupplem ented by offboard data

6 Real tim e m ultivehicle cooperationRanged aw areness - on board sensing forlong range, supplem ented by off-boarddata

5 Real tim e m ultivehicle coordinationSensed aw areness - Local sensors to detectexternal targets (friendly and threat) fusedw ith off-board data

4 Fault/Event A daptive vehicle O ff-board A w areness - friendly system scom m unicate data

3 Robust response to real tim efaults/event H ealth/status history and m odels

2 Changeable m ission H ealth/status sensors

1 Execute preplanned m ission Preloaded m ission data; Flight Control andN avigation Sensing

0 Rem otely Piloted Vehicle Flight Control (attitude, rates) sensing; O nBoard Cam era

.

A. Methods of Modeling

The approach to helicopter modeling can be in general di-vided into two distinct methods. The first approach is known as first principle modeling based on direct physical understanding of forces and moments balance of the ve-hicle. The challenge of this approach is the complexity of the mathematical model involved along with the need for rigorous validation. The method is primarily suitable for one with a strong background in flight physics. The second method based on system identification (Tischler and Cauffman, 1992; Mettler et.al., 2002, Tischler and Remple, 2006) basically arises from the difficulty of the former ap-proach. The frequency domain identification starts with the estimation of frequency response from flight data recorder from an instrumented flight-test vehicle. The parameterized dynamic model can then be developed in the form of a li-near state-space model using physical insight and frequen-cy-response analysis. The identification can also be con-ducted in time-domain.

In what follows, the author argues that, any modeling should start from adequate basis in first-principle. In prac-tice, the above two methods can be used in an integrated scheme for developing an accurate small scale rotorcraft vehicle model for the purpose of control design. The mod-eling based on neural networks with appropriate structure and training method can be viewed as a viable alternative.


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Meanwhile, a new modeling scheme based on Linear Pa-rameter Varying (LPV) identification is attractive for RUAV application.

B. Equation of Motion of RUAV The motion of a vehicle in three-dimensional space can

be represented by the position of the center of mass and the Euler angles for the vehicle rotation with respect to the inertial frame of reference. The Euler-Newton equations are derived from the law of conservation of linear and an-gular momentum. Assuming that vehicle mass is m and inertial tensor I, the equations of motion are given by:

I

I

dVm Fdt

dI Mdt

=

=

K K

KK K

(1)

where [ ]TF X Y Z=K is the vector of external forces act-ing on the helicopter center of gravity and [ ]TM L M N=K is the vector of external moments. For helicopter, the external forces and moments consists of forces generated by the main rotor, tail rotor; aerodynamics forces from fuselage, horizontal fin and vertical fin and gravitational force. For computational convenience, the Euler-Newton equations describing the rigid-body dynamics of the helicopter is then represented with respect to body coordinate system by us-ing the kinematic principles of moving coordinate frame of reference as the following:

( )

( )

mV m V F

I I M

+ =+ =

K K KKK KK K

(2)

Here the vector [ ]TV u v w=K and [ ]Tp q r =K are the fuselage velocities and angular rates in the body coordi-nate system, respectively. For the helicopter moving in six degrees of freedom, the above equations produce six differential equations describing the vehicles transla-tional motion and angular motion about its three refer-ence axes. From here, we can express the mathematical expression for external forces and moments of the helicopter as a function of the control inputs and the vehicle states.

( ) sinX m u rv qw mg = + + ( ) sin cosY m ru v pw mg = + ( ) cos cosZ m qu pv w mg = + ( )xx yy zzL I p I I qr= ( )yy zz xxM I q I I pr= ( )zz xx yyN I r I I pq=

(4)

The forces and moments components consist of contri-bution from main rotor, tail rotor, fuselage, horizontal fin and vertical fin.

1) Main Rotor: The main rotor thrust equations are ex-pressed as:

( ) ( )2 2MR MRMR MR TT R R C = (5) where the thrust coefficient is given by

( ) 2MR MR MR MR 0MR MR 01 1 1 12 2 3 2T zC a = + + (6)

and the inflow ratio, advance ratio and normal airflow component are respectively given by

( ) ( )

( ) ( )

iMR MR0MR 22

MR MR 0MR MR

2 2

MR MRMR MR

R 2

R R

T

w z

a a az

w C

u v w

= + +

(7)

Here , a and w are solidity ratio, lift curve slope and coefficient of non-ideal wake contraction of the main rotor. The above equations must be solved iteratively to obtain the thrust. The main rotor torque can be approx-imated as a resultant of induced torque due to generated thrust, and torque due to profile drag on the blade.

( ) ( )2 2MR MR MRMR MR QQ R R R C = (8) where the torque coefficient is given by

( )MR 0

2MR MR MR 0MR MR MR

1 718 3Q D z T

C C C = + + (9)

and 0D

C is the profile drag coefficient of the main rotor. The representation of the main rotor tip path plane dy-

namics is given by


( ) ( ) Lo1 a 1 ae 1 1 eMR MRMR MRs s

s sz

a u a wa a q AR R

= + + +

( ) Lat1 ae 1 1 e LatMR MRs

s sb vb b p B

R

= +

(10)

where alat

B and longA steady-state lateral and longitu-

dinal gains from the cyclic inputs to the main rotor flap angles; lat and long are the lateral and longitudinal cyc-lic control inputs; e is the effective rotor time constant for a rotor with the stabilizer bar.

2) Tail Rotor: The tail rotor thrust can be computed by the following equation:

rTR r TRvT mY mY v = + (11)

And the normal velocity component to the tail rotor is

TR a TR TRv v l r h p= + (12)The tail rotor torque is computed using similar equations

for main rotor with tail rotor parameters substituted into the main rotor parameter.

3) Fuselage: For hover and low speed forward flight, the rotor downwash is deflected by the forward and side velocity. This deflection creates a force opposing the movement. The fuselage forces of the helicopter can be expressed as

fus fus a12 x

X S V u =

fus fus a12 y

Y S V v =

( )fus fus a iMR12 zZ S V w w = (13)

4) Horizontal tail: The horizontal tail generates lift and a stabilizing pitching moment around the center of gravity. This will also compensate the destabilizing effect of the main rotor flapping due to vertical speed. The horizontal tail fin forces and moments of the helicopter referenced to body coordinate system are

HF 0X = HF 0Y =

( )( )

HF HF HF a HF HF

2 2HF HF a HF

12

12

LZ S C u w w

Z S u w

= +

= +

(14)

5) Vertical tail: The vertical tail forces can be approx-imated by the following expression

VF 0X = ( )( )

VF VF VF VF VF VF

2 2VF VF VF VF

12

12

LY S C V v v

Y S V v

= +

= +

(15)

C. First Principle Model

The detailed equations of motion as presented previously are the basis for first principle modeling. It is a bottom-up physical modeling. A study by Weilenmann (1994) was an attempt to use first-principle approach to model the heli-copter dynamics. The modeling however was limited only to hovering condition. Some simplified version of helicop-ter model existed including the Minimum-Complexity Helicopter Simulation Math Model (Heffley and Mnich, 1988) spanning from the previous work by Heffley et.al.(1979 and 1986). In 2003, Gavrilets (Gavrilets, 2003) presented a nonlinear model helicopter based on first prin-ciple approach used for an aerobatic maneuver control. The work however does not present workable procedures for developing linear model for the purpose of control design. The step-by-step development of linear model requires the calculation of a trim condition around which the vehicle motion will be linearized. The trim conditions for the heli-copter are chosen operating points within which we solve the equilibrium condition ( , ) 0f x u =K K K by first setting the states to the values which characterize the corresponding flight condition. For the case of RUAV, the solution of trim condition is achieved through an iterative process. The no-tion of stability derivatives used in the modeling arises from Taylors series expansion of external forces and mo-ments around an equilibrium condition where only first order effects are retained. The external forces and moments are thus expressed in terms of product of derivatives and the rigid-body vehicle states and control inputs. The linea-rized equations of motion can finally be expressed in the form of state space readily usable for control synthesis. For more detail explanation, the readers are referred to (Bu-diyono, 2007b). As needed, the first principle model can also be refined by the system identification technique as presented in the following section.

D. Identification Modeling The first principle approach typically requires the detail knowledge regarding the system behavior. The use of sys-tem identification modeling either in time or frequency domain on the other hand is more practical. The system identification approach requires experimental input-output data collected from the flight tests of the vehicle. Thus the flying test-bed must be outfitted with adequate instruments


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to measure both state and control variables. To utilize ex-perimental data to build a parameterized model however, a model structure and decent initial conditions in the optimi-zation scheme would be required to achieve convergence. The model structure and its initial value in this case can be provided by prediction of first principle calculation. In structured parameterization scheme, Predication Error Minimization (PEM) method can be utilized to estimate the parameters. With the method, the parameters of a model are chosen so that the difference between predicted output of the model and the measured output is minimized with the following process.

Given the time domain description of a system:

( ) ( ) ( ) ( ) ( )y t G q u t H q e t= + (16) and by observing the input (u) and output (y) data, the

error, e(t) can be computed as: 1( ) ( )[ ( ) ( ) ( )]e t H q y t G q u t= (17)

PEM uses optimization to minimize the cost function, defined by:

2

1( , ) ( )

N

Nt

V G H e t=

= (18) The result of combined first principle and identification modeling is illustrated in Fig. 2. The figure shows the for-ward velocity flight data (solid thick line) compared with the first principle model (solid thin line) and identification model (dashed line). The figure shows that the fitness ratio of the flight data for first principle and identification model is 19.87% and 24.34% respectively.

Figure 2: Comparison of first principle and ID result

TABLE II

STABILITY DERIVATIVES COMPARISON BETWEEN FIRST PRINCIPLE

PREDICTION AND IDENTIFICATION

First Principle Pre-

diction Identification

Yv -0.3471 -0.8652 Yr -16.5191 -16.286

Yb1s 10.1395 134.74 Lu -0.0106 -0.03 Lw 0.1098 0.0703 Lv -0.2486 -0.217 Lp -40.8739 1.3026 Lb1s 408.5485 320.53 Nw 1.0103 1.3669 Nv 2.5045 2.1817 Np 0.1406 -1.2065 Nr -0.9758 -0.695

Ba1s / e

0 0.0656

Further comparison between the first principle prediction and identification result is given in Table II.

E. Linear Parameter Varying Identification All previous modeling schemes boil down to the develop-ment of linear model associated with a certain flight condi-tion as shown in Fig. 3. The design of global nonlinear control is then predicated on the notion of gain scheduling. The drawback of this approach is that control designs based on linearized dynamics might become deteriorated when it is applied beyond the vicinity of equilibrium. In contrast, LPV control technique explicitly takes into account the change in performance due to real-time parameter varia-tions. Therefore, this control technique gives a promising potential in designing control systems which is robust over the entire operating envelope.

Hover

Accelerate

Cruise

Deccelerate

Maneuvers

Ascend

Descend

Piourette

RUAVsFLIGHTCONDITIONS

Figure 3: RUAVs flight conditions


0 50 100 150 200 250 300-5

0

5

10

15

20

25

time(seconds)

u(m

/s)

Estimated ResponsePlant Response

Figure 4: Result of LPV identification for forward speed

The LPV identification scheme employs recursive least square technique implemented on the LPV system represented by dynamics of helicopter during a transition. The airspeed as the scheduling of parameter trajectory is not assumed to vary slowly. The exclusion of slow para-meter change requirement allows for the application of the algorithm for aggressive maneuvering capability without the need of expensive computation. Fig. 4 shows the result of LPV identification for varying forward speed. More detail account can be found in (Bu-diyono, 2008b).

IV. Simulation, Control and Guidance

To date various control techniques have been designed for rotorcraft vehicles ranging from classical sin-gle-output-single-output PID controller (Shim, 2000) to nonlinear (Koo, 1998; Boussios, 1998; Devasia, 1999; Buskey et.al., 2001, Harbick, 2004) and from non-aggressive flight (Corke et.al., 2000; Castillo et.al., 2005) to aggressive flight (Gavrilets et.al., 2001). To cover a wide region in the flight envelope, a gain schedule tech-nique is typically employed as in Shamma and Athans (1991). A control using state-dependent Riccati equation was proposed by Bogdanov and Wan (2003) and Bogdanov et.al.(2003). The scheme was implemented on X-Cell heli-copter (Bogdanov et.al, 2004). A control synthesis based on behavioral approach was suggested by Fagg et.al. (1993) and Buskey et.al. (2002,2003). Fuzzy (Jang and Sun, 1995) and adaptive control have been also synthesized for control of RUAV (Hovakimyan et.al. 2000; Johnson and Kannan, 2002; Kannan and Johnson, 2002; Kim et.al., 2002; Kutay et.al., 2002, Sanchez et.al., 2005). Bagnell and Schneider (2001) proposed a control using reinforcement learning. A Lyapunov control design was proposed by Mazenc et.al.

(2003). Overall, there exists a tendency in the area of RUAVs that more research has been done in control design methodolo-gies than in developing dynamics model. The author argues that modeling is prerequisite of good control design. In order that a control system can be successfully designed and implemented for a vehicle (system), the dynamics cha-racteristics of the vehicle must be well-understood. In line with this argument, Mettler (2003) viewed that the tenden-cy to get around modeling efforts by searching for perfect control methodology is not productive and can even lead to inaccurate or misleading conclusions regarding the appli-cability or performance of certain control techniques. Flight simulation based on the developed model can be used to complement flight testing (Johnson et.al., 1996; Johnson and DeBitetto, 1997; Munzinger, 1998; Perhinschi and Prasad, 1998; Johnson and Fontaine, 2002; Johnson and Mishra, 2002; Lee and Horn, 2005). Guidance can be viewed as the most outer loop of multi-loop control sys-tem.

A. Simulation environment for UAV

Research in control engineering regularly produces new theoretical insights and algorithms that promise substantial improvement over the state of the practice. However, it is only a small fraction of this research that ultimately sees practical application (Samad et.al, 2004). The area of con-trol for UAVs is not an exception. The need to close the gap between theory and application of control to UAVs in real operating conditions has been addressed by creating simu-lation environment where actual time-dependent signals are taken into account. Implementation and testing of control systems by a hardware-in-the-loop (HIL) simulation is in-creasingly being required for the design as it becomes a very versatile tool in acquiring real data without taking a risk of losing any expensive instrumented UAVs. HIL si-mulation is characterized by the operation of real compo-nents in connection with real-time simulated components. Usually, the control system hardware and software is the real system while the controlled plant can be either fully or partially simulated. The high-confidence control can be achieved by developing increasingly higher fidelity models and simulations through successive improvements. It should be ensured that the plant model is a sufficiently ac-curate approximation of reality and that assumptions about disturbances and the operational environment are valid. The implementation of HILS for various RUAVs at Smart Robot Center (Konkuk University) is illustrated in Fig. 5.


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PC104

long

lat

col

ped

,,

Motiontable

IronBird

HardwareIntheLoop(HIL)Simulator

IMU

HostPC xPC

RS232 RS232

Windows

FlightcontrolexecSensorsignalprocessorGCScommunication

FlightGearCockpitview

Matlab/Simulink6DOFheli nonlinearmdlRTW/XPCTargetappl.

HealthmonitoringInteractiveautopilotingGroundcontrolGUIHighlevelcontroller

RTsimulink executionBidirectionalcommunication

Figure 5:Simulation environment for UAV control synthesis

B. Control Synthesis Given the sufficiently accurate model, the control synthesis of RUAV can be conducted and validated within real-time simulation environment. Various control techniques have been developed thus far in Budiyono (2005a, 2005b) and Budiyono et.al. (2004, 2005, 2007a). Referring to the tax-onomy of flight conditions of RUAV (Fig. 3), the control design can be classified into the following different ap-proaches:

1. Classical control. Since the problem of RUAV control is a MIMO problem, the design procedure of classical approach is to be conducted in cascaded multi-loop SISO system starting from the innermost loop out-ward. The cascaded multi-loop SISO approach how-ever has limitations in its implementation. To imple-ment this control approach for a small scale helicopter, a pitch and roll attitude control system is often subor-dinated to a, respectively, longitudinal and lateral ve-locity control system in a nested architecture. The re-quirement for this technique to work is that the inner attitude control loop must have a higher bandwidth than the outer velocity control loop. While this is va-

lid for a relatively large unmanned helicopter such as Yamaha R-50, for a class of high-performance heli-copters, such as the X-Cell 60, or helicopters where this bandwidth separation is not sufficient, a simulta-neous design will be necessary (Mettler, 2003). The simultaneous design is provided by modern control synthesis.

2. Modern MIMO control. To control a model helicopter as a complex MIMO system, an approach that can synthesize a control algorithm to make the helicopter meet performance criteria while satisfying some physical constraints is required. To address a MIMO problem, LQR and H are the most popular control design procedures. These methods however also have drawbacks that can inhibit a practical implementation. They include dealing with higher than necessary order of controller, non-existence of formal parameter tun-ing and weight selection procedures, possible exclu-sion of good controllers, and difficulty in integrating state variable constraints (Manabe, 2002).

3. Algebraic control. The CDM is one of such ap-proaches where control design process is based on


coefficient diagram representing criteria of good de-sign. The use CDM thus far has been limited to SISO or SIMO applications. Some trial designs for MIMO have been made (Manabe, 2002), but formal design procedures to implement CDM for MIMO has not been established yet. The typical approach in solving MIMO problem thus far has been to decompose MIMO problems into series of SISO or SIMO prob-lems and proceed with design by standard CDM. The first attempt that demonstrates a successful imple-mentation of CDM-based LQR technique without the need of decomposing a MIMO problem into a series of SISO or SIMO problems was presented in (Bu-diyono, 2007). Fig. 6 shows the result of design for step response of u and w subjected to 30% parameter variation.

4. Hybrid approach. In the hybrid approach, each linear model in Fig. 3 can be considered as a hybrid auto-maton. To represent an RUAV flying over wider flight envelope therefore, the approach leads to a switching problem representing a change from one mode to another. A synthesis of switched control systems for model helicopter excited with external switches that bring changes of dynamics from hover to cruise by satisfying some constraint in the trajectories can thus be performed. Piecewise quadratic Lyapunov-like functions that leads to linear matrix inequalities (LMIs) for performance analysis and controller syn-thesis can be considered. State jumps of the controller responding to switched of plant dynamics are ex-ploited to improve control performance (Sutarto et.al., 2006). The result is illustrated in Fig. 7 showing comparison between performance of LQR and Switched Linear Control.

0 10 20 30 40 50 60 70 80 90-0.2

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Figure 7: Comparison of Switched Linear Control and LQR

5. LPV approach. The control design is performed based on the model developed through LPV iden-tification. Model Predictive Control (MPC) can be a good candidate for such an approach.

V. Emerging Technologies

Issues pertaining to increased demand for higher perfor-mance and safety have pushed the UAV design beyond conventional approaches. Some emerging technologies can be summarized in the following paragraph.

A. Bio-inspired Technologies and Biorobotics

The emerging field of unmanned system technologies largely relies on the ability of an onboard mechanism that replaces or imitates a human operator. To successfully design an unmanned system or vehicle therefore it is im-portant to study the human intelligent at all levels: reason-ing, perception, development and learning. Moreover, the compelling need to learn from nature stems from the fact that although the present conventional approach to engi-neering design may exceed nature in some regards, they are not superior to many designs in nature. Using conventional approach, present day UAVs can perform different control functions including altitude and speed hold, obstacle avoidance, terrain following navigation, and autonomous landing. Flying insects can perform all those and beyond, remarkably well using ingenious strategies for perception and navigation in three dimensions. Insects infer distances to potential obstacles and objects of interest from image motion cues that result from their own motion in the envi-ronment. The angular motion of texture in images is de-noted generally as optic or optical flow. Computationally, a strategy based on optical flow is simpler than is stereosco-


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py for avoiding hazards and following terrain (Thakoor, S. et.al. , 2003).

Recent studies also demonstrate that insects can perform extreme maneuvering capabilities far beyond those achieved by conventional UAVs. Flapping wing, morphing wing, formation flight, neuro-control and swarming are just a few examples of natural phenomena much related to UAVs advanced design features. More research should be consistently conducted for harvesting design principles from nature that would extend present UAV technologies out of its conventional boundaries.

B. Multi UAV Systems

One primary feature of high autonomy UAVs is their ability to perform coordination and cooperation functions. This capability is termed Level 5 and 6 in Table 2. Research in this area (collaborative sensing and exploration, synchro-nized motion planning, and formation or cooperative con-trol) has been gaining more interests in recent past as shown for example in (Seiler, 2001) and Mot et al. (2002a, 2002b). A particular class of tasks for such mul-ti-agent UAV systems involve surveillance of a region and tracking of targets cooperatively. Cooperative agents are typically desired to handle a particular task with higher robustness, higher performance (faster or more accurately) or task simply otherwise unattainable by single agent. UAVs formation control can be achieved through hierar-chical (leader-follower) or non-hierarchical approach.

Cooperative multi-agents naturally lead to hybrid system abstraction. The hybrid model would capture both UAV dynamics and mode switching logic that supervises lower level control switches. It will be desirable in this regards to have a formal tool that can verify the performance and safety of such a system where high fidelity simulation can be conducted prior to flight tests. Future research direction in multi UAVs system should address this need.

VI. Concluding Remarks

The paper discussed recent progress in the technology for unmanned aerial vehicles from the modeling, control and guidance perspectives. Dynamics of rotorcraft-based un-manned aerial vehicle is presented to describe the underly-ing principle of modeling for the control synthesis. The modeling based on first principle, system identification and LPV identification is presented briefly for illustration. A number of major trends in aerial robotics are discussed: state estimation algorithm, SLAM, vision for guidance, integrated modeling, maneuver automaton and safety veri-

fication. Future challenges for advancing aerial robotics technology will be pivoted on exploitation of biomimetic principles for achieving higher peformance and develop-ment of formal model and analysis tool to synthesize col-laborative aerial robotics behavior.

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